The purpose of this study is to characterize host and viral interactions involved in viral development. The model system employed is the bacteriophage lambda and its Escherichia coli host. The basis of the approach is the isolation of bacterial mutants that are defective in supporting the activity of specific phage functions. Genetic and physiological characterization of these mutations should permit us to identify the nature of the host functions involved. The bacterial mutants, in turn, will be used to isolate phage mutants that can overcome the blocks on phage growth imposed by the specific host mutation. The determination of how the phage can overcome these imposed blocks on development should yield information on the nature of the normal regulatory mechanisms expressed by the virus. These studies should have wide applications to a number of problems. (1) Information gained on viral-host interactions will be useful in understanding such varied viral associated pathological processes as viral induced malignant transformation and lysogenic conversion. (2) Defining the factors influencing gene expression by phage lambda should prove useful in designing conditions for optimizing gene expression by lambda vectors carrying exogenous DNA.